Activated carbon basics
Activated carbon is a highly porous, highsurface- area adsorptive material with a largely amorphous structure. It is composed primarily of aromatic configurations of carbon atoms joined by random cross-linkages.
Activated carbon differs from another form of carbon – graphite ; in that activated carbon has sheets or groups of atoms that are stacked unevenly in a disorganized manner. The degree of order varies based on the starting raw material and thermal history. Graphitic platelets in steam-activated coal are somewhat ordered, while more amorphous aromatic structures are found in chemically activated wood.
Raw materials
Almost any carbon-containing material can be used to produce activated carbon. In practice, economics and target product properties are the determining factors in the selection of raw
materials. The base raw material has a significant impact on the final product properties, including pore size distribution and volume, hardness and purity.
Most commercial activated carbons are manufactured from the following raw materials:
• Coal (anthracite, bituminous, subbituminous, lignite)
• Coconut shell
• Wood Some types of activated carbon are produced from less conventional raw materials, such as peat, olive stones, fruit pits, petroleum coke, pitch, synthetic polymers, scrap tires and waste cellulose materials. Raw materials may undergo pre-processing steps to control size, form and other properties. They may be crushed, milled, briquetted or mixed with binders and extruded prior to activation.
Application types
Activated carbon sorbents are used in two broad application classes: vapor and liquid purification. Within each class are examples of two types of fluid-sorbent contacting. These are PAC dosing and GAC/EAC packed columns. Beyond purification, activated carbons are used in a number of specialized applications.
Activated carbon sorbents are used to purify a wide range of liquid systems, including potable water, wastewater, industrial process water, chemicals, pharmaceuticals, foods and beverages. Removal of taste and odor contaminants and harmful pollutants from potable water is one of the main purification applications.
Activated carbon is used to remove precious metal catalysts after synthesis of pharmaceutical active ingredients. Liquid sugar decolorization is one of the earliest activated carbon applications and remains important today. Activated carbon is used to remove undesirable taste compounds and color precursors from processed fruit juices. Color removal from natural-gas liquids is a more recent application. Activated carbon application in sediment remediation is covered in more detail below. Non-purification applications of activated carbon include gold recovery from cyanide leaching solutions, storage of gases, such as hydrogen and methane, and electrodes for doublelayer capacitors and catalysts.
In conclusion, activated carbon is a sorbent with wide-ranging uses in the purification of vapor and liquid systems, as well as specialized uses in fuel storage, catalysis and electrochemistry. Activated carbon can be tailored for specific applications by a combination of raw material selection, activation process conditions and post-processing, including shaping and chemical impregnation. Adsorption performance is driven by pore size and volume distribution, and other factors, such as durability and particle size, influence the choice of activated carbon for a specific application. Activated carbon can be used in liquid or vapor applications in either PAC or GAC/EAC form. Biogas purification and sediment remediation are two recent applications that highlight the versatility of activated carbon sorbents for purification applications.
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